On Thu, Jul 29, 2021 at 02:57:12PM -0400, John Levine wrote:
Since there are already IDs for IMAP and POP3, it seems we are down to
3 (+SMTP) or 4 (+SUBMIT) unless we want to break existing applications?
I'd prefer to see "SUBMIT" as a separate ALPN value from "SMTP".
Given that clients often don't know whether they're talking to an SMTP or a
In fact they almost always do, because "SUBMIT" generally requires
authentication, and SMTP relay does not, and the separate ports also
and servers don't know which one they are or maybe both, one ALPN would be
Submission servers are explicitly configured to accept outbound mail,
and it has been best practice for quite some time to not implement
submission on port 25, leaving that to 587, and more recently also 465.
It's still pretty common for a server to offer SMTP on port 25, which
turns into submission if the client authenticates.
This is a no longer recommended and diminishing practice.
- An MUA can always use the "SUBMIT" ALPN value.
- An occasional MTA that is configured to relay outbound mail
via a submission server, and has been updated to use ALPN,
can be configured to use "SUBMIT" when appropriate.
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